b
(A) \(\left[ Ni \left( H _{2} O \right)_{2}( en )_{2}\right]^{2+}\)

(B) \(\left[ Ni \left( H _{2} O \right)_{4}( en )\right]^{2+}\)

(C) \(\left[ Ni ( en )_{3}\right]^{2+}\)

en is SFL (strong field ligand)

As the number of en (strong ligand) increase splitting also increases.

So, \(\Delta_{0}\) increases.

i.e. maximum energy will be absorbed in case of option \(C.\)

So the order is \(C\,>\,A\,>\,B\)